The present invention relates to a light fiber of directional side light extraction type capable of extracting a light from the sidewall and a process for producing the same. More particularly, it relates to a light fiber of directional side light extraction type capable of extracting a light in a specific direction from the sidewall and a production process thereof.
Discharge tubes such as fluorescent lamp emit a visible light of a particular wavelength region, they are ordinarily used in the application fields for lighting. In the case where the discharge tubes are a neon bulb, they are used to demonstrate an advertisement or decoration or the like in the form of the so-called neon sign.
The discharge tubes are luminescent with the application of electric voltage. Generally, the discharge tubes generate heat. In such a case, it is necessary to use the discharge tubes with due regard to an electric leakage and the generation of heat. For example, the use of discharge tubes is substantially impossible for under-water lighting or demonstration.
In recent years, however, a light-emitting device comprising a light source located at a distance from the area illuminated therewith has attracted attention to achieve the lighting and demonstration as set forth above. In the case of those light-emitting devices, a light fiber is installed in the vicinity of an area to be illuminated, away from the light source, so as to give the illumination of a desired light. Generally, the light fibers comprise a core in the central portion and a clad having a refractive index lower than that of the core on the periphery of the core. A light can be injected into one end and transmitted to the other end of thus configured light fiber.
Known among the light fibers is a light fiber of directional side light extraction type which can extract a light from the sidewall. Those light fibers of directional side light extraction type can extract parts of a light from the sidewall through the clad to the outside, when the light is injected into one end and transmitted to the other.
For example, a light fiber of directional side light extraction type which is disclosed in JP-A-6-118244 as a light-illuminating plastic optical fiber can leak lights from the whole or a designated part of the sidewall. More in detail, this light fiber is provided with a core and a clad which are made of transparent materials, respectively. The core comprises a polymer mainly composed of a polymethyl methacrylate. The clad is made of two different parts one of which is composed of a vinylidene fluoride-based copolymer comprising 50 to 90% by mole of vinylidene fluoride and 10 to 50% by mole of tetrafluoroethylene, and another part comprises at least one of the other polymers having a refractive index higher than that of the vinylidene fluoride-based copolymer, preferably a polymer which is the same as or close to the core-forming polymer. Thus, in portions of the clad wherein the vinylidene fluoride-based copolymer and the other polymer are contained, the clad and core are compatible each other on the interfacial boundary between them. In the interface between the core and the clad which are compatible, the light can not transmit in the core with total reflection, but scatters about within the clad; as a consequence, the light is extracted to the outside.
Furthermore, JP-A-10-142428 discloses a light-illuminating rod capable of extracting a light from the sidewall to a specific direction. This light-illuminating rod is basically provided with a flexible rod member serving as a core in the central portion and a transparent clad layer having a refractive index lower than that of the core and bonded to the outer periphery of the flexible rod member. A light diffusive and reflective film comprising a light transmitting polymer in which light diffusive and reflective fine particles are dispersed is present locally therebetween along the longitudinal direction of the rod member. This light diffusive and reflective film reflects at least parts of a light injected from one end of the light-illuminating rod member, diffusing and reflecting it to the outside from the radiation surface of the clad in the front of said light diffusive and reflective film.
On the other hand, JP-B-4-70604, and the like disclose inventions relating to light-illuminating rods comprising the inflexible rod members. A hardly flexible material such as quartz glass or optical glass is used as a rod member in those inventions. Furthermore, in those inventions, there is disposed in the shape of a stripe on the outer periphery along the longitudinal direction of the rod member a light diffusive and reflective film formed from a light transmitting polymer as a binder and fine powders having a refractive index higher than that of the rod member which have been incorporated into said polymer.
In the case of the light-illuminating plastic optical fiber disclosed in JP-A-6-118244, the two different polymers forming the clad are incompatible each other and caused a phase separation. Those two incompatible and phase-separated polymers have as little a refractive index difference as about 0.25 at most. In this light-illuminating plastic optical fiber, therefore, the clad cannot exhibit a light diffusive and reflective effect good enough to extract a highly bright light in a specific direction from the sidewall efficiently if an ordinary light source is used.
Generally, the light diffusive and reflective film disclosed in JP-A-10-142428 is formed by applying a coating material containing a light transmitting polymer and light diffusive and reflective fine particles to a rod member. Alternatively, if the light diffusive and reflective film is a light diffusive, reflective and adhesive film, it is laid directly on the rod member. However, those light diffusive and reflective films have a thickness of from 10 xcexcm to 110 xcexcm at most in the case of commercialized products because of various restrictive factors at the time of the production. Thus, at least parts of light which is injected into the flexible rod member from one end of the light-illuminating rod is leaked through those light diffusive and reflective films to the outside. Therefore, there is a fear that a highly bright light can hardly be extracted in a specific direction from the sidewall in high efficiency.
It is theoretically possible to provide the light-illuminating rods disclosed in said JP-A-publication with a light diffusive and reflective film having a high thickness. However, the attempts to do so are accompanied by various inconveniences. That is, if one intends to form a light diffusive and reflective film having a specific thickness by repeatedly applying a coating material to the core portion of a rod member, the process requires many man-hours and thus economically unacceptable. Although it depends upon the nature of coating materials to be used, a rod member having a sufficient flexibility as desired will not always be obtained. In the case where a light diffusive and reflective film is transferred and adhered to the core portion, a clad layer is formed on top of thus obtained film after the transfer and adhesion of said film. As a consequence, a heat-shrinkable type material may hardly be used. If it is used, a rod member having a cross section substantially in the shape of a circle can not be produced since the light diffusive and reflective film portion is partially protruded unlike in the case of the ordinary light-illuminating rods.
Furthermore, when they are used, the light-illuminating rods are generally mounted on a commercially available holder-rail having a reflective surface on the internal surface, such as U rail (white) manufactured by Sumitomo-3M, with the intentions to improve the directivity of the lights to be extracted to the outside or the like. Since the holder-rail is made to have the cross section in the shape of a circle which is the most popular cross section among the light-illuminating rods, it appears likely that the light-illuminating rods having a thick and protruded light diffusive and reflective film portion are not housed surely in the said holder-rail.
Therefore, an object of the present invention is to provide a light fiber of directional side light extraction type capable of keeping the cross-sectional shape as it is desired, reducing the amount of lights leaked through the light diffusive and reflective portion and extracting a light having a high brightness and a high directivity, and a process for producing the same.
The present inventors have made the intensive research in every way with a view to finding a solution in the problems as described above. As a result, it has been found that said object can be attained by forming a light diffusive and reflective portion such as a light diffusive and reflective film from, for example, a material obtained by mixing and dispersing light diffusive and reflective fine particles into a clad-forming resinous material; and said portion is formed at the inner periphery of a clad by co-extruding a resinous material only, and a mixture, for example, obtained by mixing light diffusive and reflective fine particles with a resinous material to disperse said fine particles therein at the time of forming a clad. The present invention has been completed on the basis of those findings.
According to the present invention, firstly, there is provided a light fiber comprising a core and a clad having a refractive index lower than that of the core on the periphery of the core, characterized in that the light fiber has a light diffusive and reflective portion formed by co-extrusion at least on the inner periphery of the clad;
secondly, there is provided a light fiber, characterized in that the diffusive and reflective portion comes into contact with the core;
thirdly, there is provided a light fiber, characterized in that the light diffusive and reflective portion has a thickness extending at least to the vicinity of outer periphery of the clad in a direction perpendicular to the longitudinal direction from the clad,
fourthly, there is provided a light fiber, characterized in that the light diffusive and reflective portion is formed in a predetermined thickness extending from the inner periphery surface of the clad to the core portion in a direction perpendicular to the longitudinal direction of the clad;
fifthly, there is provided a light fiber, characterized in that the light diffusive and reflective portion extends into within the core;
Furthermore, sixthly, there is provided a light fiber, characterized in that the diffusive and reflective portion is formed a linear shape or a band-like shape along the longitudinal direction of the clad;
seventhly, there is provided a light fiber, characterized in that the light diffusive and reflective portion is formed along the peripheral direction of the clad, and
eighthly, there is provided a method for producing a light fiber which comprises a core, a clad covering said core and a light diffusive and reflective portion, characterized in that a resinous material having a refractive index lower than that of the core-forming, light transmitting material, and a light diffusive and reflective material are co-extruded so as to form a light diffusive and reflective portion at least on the inner periphery of the clad.